This invention relates to a composition of matter and to a method of rotationally molding same. In another aspect this invention relates to an additive and to introducing the same into a rotational molding composition to improve the physical properties thereof. In still another aspect this invention relates to the addition of zinc stearate into rotational molding compositions containing crosslinking agents to prevent the formation of voids in the final rotationally molded article.
It is known in the art that optimum physical properties from polymers used in rotational molding processes, such as for example ethylene polymers, homopolymers as well as copolymers, require various crosslinking agents and additives to stabilize the molded polymer. It is also known that when polymers such as those disclosed above are treated with such crosslinking agents and stabilizing additives, problems develop in the final molded article due to the formation of surface pinholes. While surface pinholes can be eliminated in molded articles by omitting both the crosslinking agents and stabilizing additives, the molded articles produced therefrom are of questionable utility, inasmuch as exposure to heat, light, chemicals and temperature variations typically result in their failure because of deteriorating physical properties.
It is further known in the prior art that certain stearates are effective in reducing surface pinholes i.e. pinholes characterized as existing on the mold side of rotationally molded articles having a wall thickness of less than 1/4 inch (0.635 centimeters). See U.S. Pat. No. 3,974,114 (1976). FIG. I is provided to illustrate surface pinholes as seen by the prior art.
The above cited patent, however, does not provide a suitable stearate responsive to a problem characterized as a mechanical failure of rotationally molded articles. Mechanical failure occurs in rotationally molded articles having a wall thickness of at least greater than 1/4 inch (0.635 cm), e.g. 1/2 to 3/4 inch (1.3-1.9 cm). Rotationally molded articles of at least greater than 1/4 inch (0.635) thickness can rupture when placed in their designated end-use due to the presence of voids therein. FIG. II is provided to illustrate the problem characterized by the presence of voids in the inner wall of molded articles having a wall thickness of at least greater than 1/4 inch.
Voids are generally tear drop shaped openings having the small end of the opening adjacent the mold wall side and the large end of the opening adjacent the inner wall. The starting point of the void is dependent to some extent upon the wall thickness of the molded article. When the walls are on the order of 1/2 to 3/4 inch in thickness, the voids typically begin about 1/8 inch (0.32 cm) below the surface adjacent the mold wall side. Voids have been observed to exhibit three basic characteristics: They can be seen to terminate before reaching the inner wall; to penetrate the inner wall as will be evidenced by a cavity at the large end of the tear shaped void; or as is generally the case, to protrude from the inner wall and appear as a blister covered by a thin plastic skin of about 0.025-0.051 mm in thickness.
It is therefore an object of this invention to provide a composition of matter suitable for producing rotationally molded articles with a minimum formation of voids. Another object of the invention is the production of rotationally molded articles with a minimum formation of voids.